cgroup-util: drop 'controller' argument from cg_path_open()

Nowadays it always takes SYSTEMD_CGROUP_CONTROLLER as controller.
Let's drop it.

No functional change, just refactoring.

src/test/test-cgroup-util.c

@@ -511,7 +511,7 @@ TEST(cgroupid) {
         if (cg_is_available() <= 0)
                 return (void) log_tests_skipped("cgroupfs v2 is not mounted");
 
-        fd = cg_path_open(SYSTEMD_CGROUP_CONTROLLER, "/");
+        fd = cg_path_open("/");
         ASSERT_OK(fd);
 
         ASSERT_OK(fd_get_path(fd, &p));